The invention relates to methods for packing fibers less than 10 mm long in a case. More particularly, the present invention pertains to cartridges filled with such fibers for grenades or projectiles dispersing such fibers, for instance to act as screens or decoys in the infrared and/or millimeter wavelength range.
It is already known from U.S. Pat. No. 5,659,147 to make grenades or projectiles which when deployed release fiber particles along their paths to disperse carbon or aluminized glass fiber. Such projectiles are used for close-range defense of aircraft or armored vehicles against missiles equipped with homing radar operating in the mm range (17 to 94 GHz, and in particular 35 to 94 GHz).
The main problem in producing such cartridges is how to optimally fill them with a maximum quantity of short length fibers. The length of the dispersed fibers should be less than 10 mm to assure screening efficacy in the desired wavelength range.
The lengths of the dispersed fibers should be roughly the same as the wavelength of the radiation against which screening is desired, namely the fibers should be 3 to 6 mm when seeking infrared screening in the mm range (radiation absorbing carbon fibers also will assure infrared screening).
Generally, the shell cartridges are filled in bulk. Accordingly, the shell will not be filled optimally, and cartridge performance is not reliably reproducible due to variations among cartridge weight and fiber distribution.
Moreover, cartridges are known wherein the short fibers (of aluminized glass) are stacked like layers of small diameter (less than 40 mm for fiber lengths larger than or equal to 5 mm). Such a design improves the packing density.
U.S. Pat. No. 5,179,778 describes such a method for packing fibers into a cartridge case. This procedure employs a bush which is drawn with great force to assure radially compacting the fibers into a skein. This skein thereupon is cut into disks.
This is a complex procedure. The fibers may not rest affixed to the disks so cut off. In particular when the disk diameter is larger than 40 mm, their thickness is between 3 and 7 mm, and the disks comprise clearance receiving several pyrotechnic dispersion charges.
The objective of the present invention is a method for palliating the above cited drawbacks.
The present invention method allows packing short fibers (less than 10 mm long) in a simple and economical manner in a case, in the form of sliced disks, herein called slices, whose diameters may be large (more than 40 mm) and comprise clearances.
Thus, the objective of the invention is a method for packing fibers less than 10 mm long into a case, particularly a cartridge, the method being characterized by the following stages:
at least one set of long fibers is rearranged into at least one skein,
the skein(s) is (are) impregnated with a material having a first solidification point,
the skein(s) so impregnated are placed into a mold,
the skein(s) is (are) solidified inside the mold whose temperature is changed to the first solidification point,
when solidified, the skein(s) is (are) cut into at least two slices, each slice thickness being the desired fiber length,
the impregnation material is eliminated by raising the slices to a second temperature before or after the slices are stacked in the case.
The solidified skein can be cut when inside the mold which is fitted with recesses allowing passage of a cutting means.
Alternatively, the solidified skein can be withdrawn from the mold and fitted with a support sheath before cutting the skein into slices.
The mold may feature a semi-cylindrical cavity to impart a semi-cylindrical shape to the solidified skein, first at least two identical skeins being manufactured which then are assembled within a single cylindrical support sheath.
The mold may include a cover having a semi-cylindrical contour which makes it possible to subtend a cylindrical, axial half duct in the solidified skein.
An axial duct may be implemented by drilling through the skein or the solidified slices.
After the axial duct has been implemented in the solidified skein, but before cutting, a tube filled with the same impregnating material and in the solidified state can be advantageously housed inside this axial duct.
The support sheath may be heat-shrinking, or it may a metallic sheath.
In the latter case, the metal sheath may be a screen of which the mesh width is less than 140xcexc.
The metal case may comprise a foil enveloping the skein and soldered edge to edge.
The array of the long fibers into a skein can be implemented by winding a long fiber between two pins mounted on a support.
Alternatively, the rearrangement of the long fibers into a skein may be implemented by winding a unidirectional web on itself.
The solidifying material may be water, or it may contain water.
The solidifying material may have a solidification point higher than 0xc2x0 C. and a melting or boiling point less than 150xc2x0 C. It may be a wax.
The fibers may be carbon fibers or glass fibers covered with a conducting material such as aluminum, or conducting organic fibers.